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Juerg Feldmann

Fortiori Design LLC
Posts: 1,530
I got  a few  mails   sent to me  with the question, why I would be  so " negative" about NIRS  and lactate.
I am not sure  whether critical  can be called negative.
Secondly  I was  and many do not know the story how  small point of care  lactate analyzer  actually moved  to  north America , but  I collected  lactate sample s like people  collect stamps. (far over 1000'000 sampling )
 So not negative  at all but somewhat critical.
 I like to explain that  as  short as possible.
 a) Lactate is a systemic information gathered  somewhere  at a certain time. The value you will gather  does not tell you  any thing more, that there was some where in the body  sometimes  before  you  sampled  the blood a  metabolic  situation occurred , where  someone part created  lactate(. Aerobe  or  " anaerobe  )
 We  have no clue  from the sample  how much  lactate we created  and where , and we  have no clue  how  much lactate  from the place  where it was  created  made  it to  the sampling place  and how much was "used" or  moved  towards  another area..
 We  not even know from a single sample, whether it is  a  situation, where the next sample will show a higher value or perhaps  a lower value  or a   +-    same value..

 2. Nirs  gives  you a  local  feedback on O2  demand  and  supply  and in some cases  when we use a  second  NIRS  at a  less involved  muscle we  can get a  systemic  trend  as well.
. Now this  has  some limitations as well. You may see a very low SmO2  value  close  to zero, but the athlete   is  still moving absolutely easy .
 You may see that  in  a hard  workout  some  muscles  may de-saturate ,  but than as you go harder  may  reach a  flat  SmO2    or even may increase  SmO2.
 So  the trend is  crucial   and not  an absolute number. When we  combine SmO2  with tHb we have  more information  why SmO2  did  what we  see.
. One  easy to explain  example is  in cycling.  You may have some  athletes  they  show a very minimal  or surprisingly small desaturation trend   when using  the VL. But they create  a lot  of  wattage ???
 Before  searching too far  it could be , that they use much more  hip extension  instead  of  leg extension  and  therefor  show a  very  minmal  SmO2  drop. There  can be as well other reasons.

 In cycling  you may in fact  switch  muscle groups  as you go harder and harder.
 So  for example your  calf  muscle may  initially work  nicely  an than  will be  phased  out to a much smaller  %  of  performance,as you go harder.
 Or  you may switch  from leg extension  , quadriceps  parts  to more hip extension  like  using the gluteal part. this changes  will allow  to maintain a longer , higher intensity  and is  easy to be used  by  perfect coordination in top athletes. Whether they do this as planned  or whether it just happens  depends  on the athlete.
. This  switch  can create  lactate in one part of a  muscle but not in another part . So  when you compare   HHb  or  O2H  or SmO2     and trends  like for break points  you may find sometimes  one   and sometimes  not at all, but    the chances , that lactate  and BP  of  NIRS  will have  any  regular appearance  is  limited.
. Here a very short  summary on  what I mean  from a  great study.

Med Sci Sports Exerc. 2009 Jun;41(6):1277-86. doi: 10.1249/MSS.0b013e31819825f8.

Changes of pedaling technique and muscle coordination during an exhaustive exercise.

Dorel S1, Drouet JM, Couturier A, Champoux Y, Hug F.

Author information

  • 1Research Mission, Laboratory of Biomechanics and Physiology, National Institute for Sports (INSEP), Paris, France.



Alterations of the mechanical patterns during an exhaustive pedaling exercise have been previously shown. We designed the present study to test the hypothesis that these alterations in the biomechanics of pedaling, which occur during exhaustive exercise, are linked to changes in the activity patterns of lower limb muscles.


Ten well-trained cyclists were tested during a limited time to exhaustion, performing 80% of maximal power tolerated. Pedal force components were measured continuously using instrumented pedals and were synchronized with surface EMG signals measured in 10 lower limb muscles.


The results confirmed most of the alterations of the mechanical patterns previously described in the literature. The magnitude of the root mean squared of the EMG during the complete cycle (RMScycle) for tibialis anterior and gastrocnemius medialis decreased significantly (P < 0.05) from 85% and 75% of Tlim, respectively. A higher RMScycle was obtained for gluteus maximus (P < 0.01) and biceps femoris (P < 0.05) from 75% of Tlim. The k values that resulted from the cross-correlation technique indicated that the activities of six muscles (gastrocnemius medialis, gastrocnemius lateralis, tibialis anterior, vastus lateralis, vastus medialis, and rectus femoris) were shifted forward in the cycle at the end of the exercise.


The large increases in activity for gluteus maximus and biceps femoris, which are in accordance with the increase in force production during the propulsive phase, could be considered as instinctive coordination strategies that compensate for potential fatigue and loss of force of the knee extensors (i.e., vastus lateralis and vastus medialis) by a higher moment of the hip extensors.


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